Hmm tough situation! If you earn that much less than your ex you would definitely be eligible for child support. It will be a significant amount if he earns good money.
However you have a current arrangement so maybe best to chat with ex first? Perhaps you could ask for a 75/25 split? I would say that would still be better for him rather than child support. I am in a very similar situation, we have a private arrangement whereby ex pays me a monthly amount plus we split the cost of big stuff. He is hassling me to just split all costs but child support estimate is a significant amount per month and as much as he doesn't like it, the CSA takes into account that Mums lost income and career progression and thus equal pay, when they stayed at home to look after babies.
Personally I don't entirely agree with the way CSA calculate child support but I also think you are definitely entitled to more than what you are currently getting. Good luck.